This invention relates to structures for protecting color television picture tube bulbs, and bulb components, from implosion, primarily. A conventional color television picture tube has an evacuated glass bulb which includes a faceplate with a rearward flange and a funnel sealed to the faceplate flange along a planar sealing interface. The faceplate has a concave inner surface upon which is deposited a luminescent phosphor screen. Due to the high vacuum in the bulb, several tons of atmospheric pressure are exerted on the faceplate and funnel, causing the bulb to be susceptible to implosion. (The term "implosion" is defined by Underwriters Laboratory Incorporated as a "rapid and sudden inward bursting of a high-vacuum glass envelope.") It is of the utmost importance in the interest of safety to prevent the bulb from violently imploding should it, for example, be struck by a heavy missile.
There have evolved a number of approaches to implosion protecting color CRT's (cathode ray tubes) of the described type having a conventional bulb (with a flanged faceplate). A first approach seeks to confine or restrain the shards of a fractured bulb. One type of system which implements this approach is referred to as a "rimbond" system. FIG. 1 shows schematically a color CRT bulb including a faceplate 1 of the conventional (flanged) type on which is mounted a rimbond implosion system. The rimbond system includes a scalloped metal frame 2 which surrounds the flange 3. A small gap provided between the frame 2 and the faceplate flange 3 is filled with a cement 4--typically an epoxy resin. In a rimbond system, the frame is not under tension. The cemented frame confines the glass shards 5 of a shattered faceplate long enough to permit gradual (and therefore nonviolent) devacuation of the bulb. If, however, the amount and distribution of cement is not just right, or the structural integrity or design of the frame is deficient, the periphery of the bulb will not be adequately confined. Upon application of an impact force to the faceplate ("F" in FIG. 1), the periphery of the faceplate moves outwardly (distance "a" in FIG. 1), permitting the shards 5 of a fractured faceplate to rapidly unlock and collapse inwardly (FIG. 2). The result is apt to be a violent implosion of the bulb.
Although the mechanics of implosions in television cathode ray tube bulbs are not fully understood, it is known that the strength, and the stiffness of the frame--(bending and torsional stiffness, as well as hoop stiffness) is very important to the achievement of implosion protection. Secondly, and perhaps of even greater importance, is the structural coupling between the frame and the bulb. As noted, in a rimbond system, retention of shards to prolong devacuation is indispensable. If shards are permitted to move relative to the frame or each other, they are apt to rapidly unlock and collapse. Even if we assume the frame to be immovable (which it isn't), if the loads (radial and axial, primarily) and moments exerted by the shards upon implosion are but weakly coupled to the frame through a relatively soft cement, a violent implosion is apt to result.
Structural coupling is achieved in rimbond systems by the cement introduced between the frame and the bulb. The bond strength of epoxy to glass (an important factor in the structural coupling between frame and bulb) is not as great as desired, particularly as the epoxy ages. Nevertheless, epoxy-type cements, in spite of their very high cost and degradation upon aging, are the most commonly used materials.
It is believed that if each frame could be custom fitted to its associated bulb, thereby minimizing the amount of cement required, better implosion protection might be achieved. This is because the cement (typically an epoxy) with a Young's modulus of only a few hundred thousand psi, unfortunately has a cushioning effect on implosion-induced loads on the frame. Form-fitting each frame to a particular bulb is, however, ruled out by its prohibitive cost. In practice, the frame is sized to be slightly larger than the largest (relative to "bogie" or nominal) bulb. Thus the cement-fitted gap between the frame and the smaller bulbs (relative to bogie) is quite large. This fact results in production tubes necessarily having cement thickness larger, on the average, than might be desirable to achieve the best possible implosion protection.
The cost of the frame, and particularly of the epoxy cement, are obvious drawbacks to rimbond systems. Also, such systems significantly enlarge the size of the bulb and tend to have less long term reliability (due to degradation of the epoxy cement) than is desirable. Exemplary U.S. patents illustrating rimbond systems are U.S. Pat. Nos. 3,485,407; 3,558,818; 3,412,203 and 3,835,250.
A second basic implosion protection approach involves placing a high compressive pre-load on the bulb. FIG. 3 is a highly schematic and exaggerated depiction of a "tension band" execution of this second basic approach; FIG. 3 is a sectional view along a diagonal. In FIG. 3 a tension band 8 is placed on the faceplate under very high tensive load--e.g. 1500-2000 pounds. The broken lines represent the faceplate configuration before pre-load; the amount of pre-load shown in FIG. 3 is exaggerated. It is common to underlie the band 8 with a layer 9 of tape (friction, vinyl, or the like) to cushion mold lines or other irregularities in the glass. Since all faceplates today are rectangular, most of this load is applied in the corner regions of the faceplate. The operation is not primarily one of confinement, as in rimbond systems, but one of pre-stress. By compressively pre-loading the faceplate corners, the implosion-induced tensive stresses in the faceplate must first overcome the compressive pre-stresses in the faceplate before the faceplate will fracture.
This latter approach is popular commercially because of its low cost, but has a number of shortcomings. It suffers from a criticality in placement of the band. E.g., a misplacement of the band of one-eighth inch may destroy the implosion protectability of a tension band system. It suffers also from its complete impotence when applied to bulbs of certain sizes and configurations. Like rimbond systems (but to a lesser degree), tension band systems add undesirable bulk to the finished bulb. Structural coupling of the band 8 to the bulb is nonexistent or very slight at all loads on the bulb periphery except the corners, and even there it is reduced somewhat by the cushioning effect of the underlying layer 9. Numerous patents have been issued on various aspects of tension band systems; e.g. U.S. Pat. Nos. 3,818,557; 3,456,076, 3,556,306; 3,597,537; 3,777,057; 3,845,530; and 3,890,464.
Another approach to implosion-protecting color CRT's with conventional faceplates involves the use of a two-part frame and a tension band to constrain the frame. The frame comprises a pair of "C"-shaped half frames. A viscous cement (an epoxy, typically) is usually applied to each of the half-frames; the half-frames are then placed around the faceplate flange with their ends abutting or overlapping. Before the cement has set, a tension band is drawn up tightly around the frame and faceplate flange. It is common to use a lighter weight tension band than is used in a pure tension band system.
In such systems, the primary attribute of a tension band (pre-loading of the faceplate) is lessened by the cushioning effect of the underlying cement and half-frames. The systems have elements of cost of both rimbond and tension band systems, and are thus more costly than tension band systems, but do not fully utilize the implosion protection principle of either. For an example of this latter type of system, see U.S. Pat. No. 3,845,530.
Yet another basic approach utilizes a secondary faceplate which is bonded over the outer surface of the functional faceplate so that should a destructive impact occur, the two faceplates fracture with their own random pattern of cracks. The result is an overlapping of bonded-together shards which very effectively restrain the scattering of glass. The major disadvantage of this secondary faceplate system is, however, its very high cost.
U.S. Pat. No. 3,647,960--Takemoto et al discloses still another implosion protection system for a color CRT of the type having a conventional bulb with a flanged faceplate. The implosion system of Takemoto et al comprises a series of closely spaced, mutually insulated turns of wire wrapped (with or without tension) around and adhered to the flange of the faceplate. See also Powell et al--U.S. Pat. No. 3,519,161 in this connection.
U.S. Pat. No. 3,166,211--Stel et al discloses an implosion protection system for a CRT which utilizes as one component a fiber-impregnated sheath on the exterior of the CRT bulb. Yet another implosion protection system for a conventional CRT bulb is disclosed in U.S. Pat. No. 3,220,593 in which a webbing material is glued to a substantial portion of the funnel and to the faceplate flange. A tension band is applied around the flange of the faceplate and over the webbing material. Other U.S. patents disclosing the use of a webbing material in an implosion protection system for a color CRT are U.S. Pat. Nos. 3,206,056 and 3,314,566.
This invention has application to color CRT's of the afore-described conventional type having a flanged faceplate. It also has application to a nonconventional color CRT bulb having a flangeless faceplate, as shown, e.g., in U.S. Pat. No. 3,912,963. The referent copending applications Ser. Nos. 639,741; 623,853; 623,954; 632,559; 718,631 and U.S. Pat. No. 4,004,092 disclose a number of predecessor implosion protection systems for a bulb of such character.
U.S. Pat. No. 2,222,197 to Engels discloses a CRT in which the bulb comprises a flangeless faceplate inset in an expanded open end of a cooperating funnel. A band allegedly providing implosion protection surrounds the outside of the funnel near the open end thereof.
The above-described prior art systems have met the industry's implosion protection needs for many years. Yet, it is believed that they do not represent the ultimate in implosion protection and cost effectiveness. As a whole, they represent a substantial burden on the cost and/or bulk of a color CRT. With a few exceptions, the prior art systems offer little or no protection of the fragile funnel and faceplate prior to the time they are assembled.